President Donald Trump traded verbal barbs Thursday with James Clapper, the former national intelligence chief, who has questioned Trump’s fitness to be in the Oval Office.

“James Clapper, who famously got caught lying to Congress, is now an authority on Donald Trump,” Trump tweeted. “Will he show you his beautiful letter to me?”

Clapper has denied lying to Congress. He says he misspoke a few years ago when he said the U.S. was not collecting Americans’ data. Documents leaked by former National Security Agency contractor Edward Snowden later indicated widespread domestic surveillance.

Clapper told CNN that the “beautiful” letter referred to notes he wrote to both Trump and Democratic nominee Hillary Clinton the night before the Nov. 8 election.

In the one to Trump — the only one that was delivered — Clapper wrote that he hoped the president would support the intelligence agencies’ practice of speaking “truth to power.” That’s a reference to sharing intelligence even if it runs counter to what a president believes or wants to hear.

On Jan. 6, before the inauguration, intelligence officials briefed Trump on their assessment of Russian meddling in the election and he was told about the existence of a dossier compiled by a former British intelligence officer. The dossier, which contained allegations of compromising personal and financial information about the president, was later released by a news organization.

“Intelligence agencies should never have allowed this fake news to ‘leak’ into the public,” Trump tweeted. “One last shot at me. Are we living in Nazi Germany?”

Clapper took offense to that tweet, accusing Trump of characterizing “us as Nazis for having delivered truth to power.”

Earlier this week, the president cut loose in Arizona, angrily renewing his fight with the press over its coverage of his comments about the race-fueled violence in Charlottesville, Virginia. He opened his remarks with talk of unity but quickly erupted in anger, blaming the media for the widespread condemnation of his response to the violence in Charlottesville at a protest organized by white supremacists.

After the speech, Clapper told CNN: “I really question his ability — his fitness to be in this office.”